Hi, welcome to the forum

Lots of questions

I'll start with the last one first. Live plants are better than fake ones. They don't have to be high-tech (that's the kind you read about injecting CO2 etc), low tech plants work just as well. These don't need fancy substrates or CO2 etc, just a bit of fertiliser. I'll come back to plants later.
Before getting any fish there are two things you need to do.
The first is find out how hard your tap water is. See if you can find that info on your water company's website. Ignore words light slightly soft, moderately hard etc and look for a number. We need that number and the unit, in full. There are half a dozen different units for hardness, but only two are used in fish keeping so we may have to convert your number.
The reason is that fish should be kept in water with a similar hardness to where they live in the wild. It is harmful to keep soft water fish in hard water and vice versa. Yes, water can be altered but it is not something I would recommend doing with your first tank.
The second is cycling the tank. This means growing the good bacteria that remove toxic chemicals from fish waste. The safest way for fish is a fishless cycle. The method for doing that is here
https://forums.thinkfish.co.uk/fishtank-filtration-and-cycling/fishless-cycling-how-to-do-it/ If you do this method, get the plants once the cycle has finished as high doses of ammonia added in one go can harm plants. Fish may make the same amount but in tiny amounts all day long not in one dose.
There is an alternative - live plants. Fish make ammonia as their waste product and plants use ammonia as fertiliser. Unlike the good bacteria, plants do not turn it into nitrite. The way to use plants is to get a lot of them, not just the odd couple. Floating plants are best for this as they get lots of light being at the top of the tank, and they can get CO2 fro the air. These make floating plants grow faster and the faster they grow the more ammonia they need.
If you decide to go down this route, get your plants and wait a couple of weeks to make sure they are thriving. Then get fish a few at a time, and test for ammonia and nitrite every day. This is to make sure the plants are taking up all the ammonia. If you do see any ammonia and/or nitrite, a large water change will reduce them to a safer level. Once you know that the plants are coping with the first batch of fish - no ammonia for at least a week - get the next batch of fish, test for ammonia and nitrite daily; and so on till you have all your fish.
Fish selection.
Bettas are not community fish despite what a shop may tell you. They should be kept alone. Once you have got into fish keeping, you'll probably succumb to multi tank syndrome, an affliction of fish keepers, characterised by the desire for more and more tanks

if this happens, think about a tank of around 25 litres for a betta.
Zebra danios may be small fish but they are incredibly fast swimmers and need a tank at least 100cm long, preferably more. They are also temperate fish, meaning they need cooler water than most tropical fish. I recommend you look round for something that doesn't need as much swimming room.
Threadfin rainbows are fine, though it does depend on your hardness. But you need at least 6 rather than 4. I should warn you that in the shops where I've seen these fish, they've only stocked males because they are the ones with the big fins and look prettier than females.
Neon tetras are also OK, if your hardness is right. Again, at least 6.
Otocinclus are also shoaling fish so you need at least 6 of those too. But they should not be added straight away; there must be algae in the tank for them to eat as many otos won't eat anything else.
Shoaling fish live in groups of hundreds in the wild. Their DNA is programmed to expect to be in a group and they will become stressed if there aren't enough of them. Stressed fish have lower immune systems and get sick more easily. It is thought that fish lose count at 6, which is why that is always the minimum number for shoaling fish - though always, more than 6 is better.
Got to go to the dentist - I'll add more later
